1) Field of the Invention
This invention relates to a hydraulic circuit suitable for use with an automatic transmission.
2) Description of the Related Art
FIGS. 5 and 6 are circuit diagrams showing two examples of an oil-pump-containing section of a hydraulic circuit for an automatic transmission. FIG. 4 is a schematic side view illustrating one example of such an automatic transmission.
According to the hydraulic circuit shown in FIG. 5, working oil in an oil pan 1 is sucked up by an oil pump 3 via an oil filter 2 arranged in a housing 11 of an automatic transmission 10. After the working oil is allowed to flow through a regulator valve 4, the working oil is divided into two portions, one being to be used as a line pressure and the other to be delivered to a torque converter and a lubricating circuit via a torque converter pressure regulator valve 5 which may hereinafter be called the "TC pressure regulator valve 5" as needed. Further, any excess portion of the working oil fed to the regulator valve 4 is returned to a suction side of the oil pump 3.
Further, a further portion of the working oil, said further portion having been divided from a delivery side of the oil pump 3, is fed to various control valves of the automatic transmission by way of an external oil filter 6 mounted on an outer wall of the housing 11 of the automatic transmission 10 (see FIG. 4).
According to the hydraulic circuit depicted in FIG. 6, working oil in the oil pan 1 is sucked up by the oil pump 3 via the oil filter 2 arranged in the housing 11 of the automatic transmission 10. After the working oil is allowed to flow through the regulator valve 4, the working oil is divided into two portions, one being to be used as a line pressure and the other to be delivered to the torque converter pressure regulator valve 5. The working oil which has flowed through the torque converter pressure regulator valve 5 is delivered via a cooler 7 to the external oil filter 6 and further to a torque converter 8.
The hydraulic circuit of FIG. 6 is also designed to have any excess portion of the working oil, which has been delivered to the regulator valve 4, returned to the suction side of the oil pump 3.
The oil filter 2, which is arranged on the suction side of the oil pump 3 and is accommodated within the housing 11 of the automatic transmission 10, is to remove solid particles from the working oil in the hydraulic circuit. The oil filter 2 makes use of a filter medium having relatively large pores because unduly small pores lead to a greater loss of the oil pump 3.
If the pores of the filter medium of the oil filter 2 are excessively large, however, the working oil delivered from the oil pump 3 still contains solid particles which may impair proper and smooth operation of each valve. As a result, it may become no longer possible to ensure the supply of the working oil in quantities as needed.
To overcome such a potential problem, the external oil filter 6 is arranged in addition to the oil filter 2 disposed inside the automatic transmission 10 so that the service life of the working oil of the automatic transmission can be prolonged and the maintenance of the automatic transmission can be facilitated.
In the hydraulic circuit of FIG. 5, the external oil filter 6 provided internally with a filter medium having smaller pores than the filter medium of the oil filter 2 is arranged on a delivery side of the pump 3 in addition to the oil filter 2, whereby the working oil from which solid particles have been additionally filtered off is supplied to the individual control valves.
According to the hydraulic circuit of FIG. 6, on the other hand, the working oil fed via the TC pressure regulator valve 5 is allowed to flow through the external oil filter 6 which is internally provided with a filter medium having smaller pores. Solid particles are hence filtered off further from the working oil. The working oil is then fed to the torque converter 8.
Incidentally, the working oil fed from the oil pump 3 to the regulator valve 4 is released to the outside when the quantity of the working oil becomes greater than that required for the regulator valve 4. The working oil so released is returned to the suction side of the oil pump 3 in both the hydraulic circuits of FIGS. 5 and 6.
The hydraulic circuit shown in each of FIGS. 5 and 6 must however be designed in such a way that, if the external filter is clogged, a relief valve provided in combination with the external oil filter is operated to feed the working oil to a downstream side. Further, the hydraulic circuit of FIG. 5 is accompanied by the problem that load to the oil pump is increased by the flow resistance of the external oil filter.